Stacked bed hydrotreating processes are known in the art. For instance, in European Patent Application No. 0,203,228 a single stage hydrotreating process is disclosed, wherein certain hydrocarbon oils having a tendency to deactivate hydrotreating catalysts by coke formation are passed over a stacked bed of two hydrotreating catalysts in the presence of hydrogen. The stacked bed comprises an upper zone containing a hydrotreating catalyst comprising a Group VIB metal component, a (non-noble) Group VIII metal component and phosphorus supported on an inorganic oxide carrier and a lower zone containing a similar hydrotreating catalyst but with no or hardly any phosphorus.
In UK Patent Application No. 2,073,770 a process is disclosed for hydroprocessing heavy hydrocarbon feedstocks, wherein the feedstock is contacted with two hydroprocessing catalysts, suitably arranged in a stacked bed configuration, which catalysts have different pore size distributions. Each catalyst comprises a refractory ceramic oxide support and as hydrogenation component one or more components of Group VIB metals and (non-noble) Group VIII metals. Promoters, such as phosphorus and titanium oxide, may also be present. Suitable heavy feedstocks are those exemplified by deasphalted atmospheric and vacuum residues, vacuum gas oils and mixtures thereof. Suitably, the process is operated under hydrocracking conditions with the carrier of the upper zone catalyst (e.g. alumina) being less acidic than the lower zone catalyst (e.g. silica-alumina).
A stacked bed process is also disclosed in U.S. Pat. No. 4,913,797. In the process disclosed, a hydrocarbon feed containing waxy components and sulfur- and nitrogen-containing compounds is first subjected to hydrotreatment and subsequently to a dewaxing treatment. The catalyst used in the hydrotreatment stage is a conventional hydrotreating catalyst, while the catalyst used for dewaxing suitably comprises a noble metal supported on a zeolite beta carrier. Between both stages a purification treatment may be carried for removing sulfur and nitrogen compounds from the hydrotreated effluent. The process may be carried out in a stacked bed mode with a bed of the hydrotreating catalyst on top of a bed of the dewaxing catalyst.
Dedicated hydrocracking processes which may be carried out in a stacked bed mode are also well known in the art. Examples of such processes are disclosed in European Patent Applications Nos. 0,310,164; 0,310,165; 0,428,224 and 0,671,457 and in U.S. Pat. No. 5,112,472. The catalysts used in these hydrocracking processes all comprise at least one hydrogenation component of a Group VIB and/or Group VIII metal supported on various carriers. However, these processes do not normally involve the use of noble metal-based catalysts, while in all processes a substantial part of the hydrocarbons boiling above 370.degree. C. is converted into lower boiling material.
Although many of the prior art hydrotreating processes employing a stacked bed configuration perform satisfactorily in terms of reducing the content of sulfur and nitrogen species present in the feed, there is still room for improvement and in particular in terms of reducing the content of aromatic species present in the feed. Particularly from an environmental viewpoint it is highly desired to reduce the content of aromatics as much as possible. Aromatic compounds reduction may furthermore also be desirable for reaching certain technical quality specifications, such as cetane number in the case of automotive gas oils and smoke point in the case of jet fuels. The present invention therefore aims to provide a process wherein hydrocarbon distillate fractions ranging from naphtha to gasoils are effectively hydrotreated in a single stage by employing a stacked bed configuration, thereby significantly reducing both the aromatics content and the content of sulfur and nitrogen species present in the feed without substantial hydrocracking occurring.